Velocity sedimentation at gravity was used to separate bone marrow cells Precursors of hematopoeitic cells, as judged both by a colony forming assay, and by staining with a monoclonal antibody specific for such cells, were separated completely from cells bearing Ly 9, a cell surface antigen of lymphoid cells. Many of the cells positive for Ly 9, lacked markers of either B lymphocytes or T lymphocytes. The presence of Lyt-2, a surface antigen of murine thymocytes and of T lymphocytes was highly correlated among thymocytes from adult mice with bright staining with fluorochrome labeled PBA (peanut lectin), e.g. there were 2 populations: PNA bright, Lyt 2+ and PNA dull, Lyt 2-. In contrast, four populations are found in fetal thymus; Lyt 2+ and Lyt 2- in the both the PNA bright and dull populations. Fluorochrome labeled lectins appear to show subpopulations of thymocytes and other T lymphocytes as judged by degree of staining measured in a fluorescence activated cell sorter. Cell sorting or lectin-affinity cell separation methods should permit separation of these cell population for functional study.